Cycles single side floor problem
First of all, it was looking like Z-fighting but it is not. I've checked twice normals, doubles, holes and everything else what comes to my mind, nothing was an issue but...

Vertices are not on the same Z location (I don't know which one) but after selecting all floor vertices - A then S > Z > 0 (it will place all of them on Z=0) everything is ok.



Little tips: this machine is hovering over the floor and use Filmic Blender for this ;)